This invention relates to an apparatus for extruding a metered quantity of a viscous material. In this apparatus, an extruder is drivingly connected to a positive displacement pump such as a gear pump so as to obviate the need for a secondary power and drive system for the gear pump.
Conventional extruders comprise an elongate, heated barrel having one or more rotatable extruder screws disposed therein. Pellets or granules of a material such as plastic are delivered into one end of the extruder barrel, and the plastic is heated and melted as it is forced along the length of the barrel by the rotation of the screws. The delivery end of the barrel usually mounts a suitable die so that the extruded material, or extrudant, may be formed into a desired cross-sectional configuration as it leaves the barrel.
Such prior extruding machines are in widespread use. However, difficulties are encountered when it is desired to extrude a product of very acurate cross-sectional configuration because the output pressure of the extruder inherently varies. Thus the pressure of the material passing through the die varies, and this in turn results in the extruded product having varying dimensions along its length. As an example, in extruding plastic film, it is common for the thickness to vary from between 6 to 10%. As a result, additional material must be extruded to ensure that the minimum thickness tolerance is maintained. This ultimately results in the film being thicker than necessary in most of its area, and a significant amount of the plastic is thus wasted.
This problem has previously been overcome by interposing a positive displacement pump as a gear pump between the extruder and the die. One example of such an apparatus for metering extremely viscous materials at very high pressures is disclosed in U.S. Pat. No. 4,336,213 to the present inventor. Other representative examples of apparatus in which a gear pump is interposed between an extruder and a die are disclosed in U.S. Pat. No. 2,680,880 to Corbett, U.S. Pat. No. 4,171,193 to Rahlfs, and U.S. Pat. No. 4,461,734 to Jones et al.
Apparatus of this type has not been driven from the extruder itself; instead, a separate motor, controller, and drive train for the pump have been provided. Such a drive system, however, is so expensive that the cost of outfitting an extruder with a positive displacement pump is much greater than the cost of the pump alone. It is, therefore, an object of the present invention to provide a positive displacement pump which can be operatively associated with an extruder at a greatly reduced cost. It is a further object of the present invention to provide a positive displacement pump which can be interposed between an extruder and a die without requiring the installation of a completely separate drive system. Other objects and advantages of the invention will be particularly identified below.